The dot matrix printers of the prior art which use a hammerbank of hammersprings with a permanent magnet and an electrical coil have provided high speed printing. The high speed printers provided by such dot matrix printers create an output which is formed from a series of dots. These printers are well known in the art and many patents describing them have been assigned to the applicant's assignee herein, namely Printronix, Inc. However, none describe the advanced magnetics and hammerbank of this invention.
During the operation of such printers, it is necessary to store energy by retaining the hammerspring against a source of permanent magnetism. The hammerspring is then released with an electrical coil overcoming the magnetic circuit retaining the hammerspring into its stored energy relationship. The result is the printing of a dot on a paper by impressing a small cylinder or rod against an inked ribbon.
The stored potential energy of the hammerspring is released when a reverse or counter flux overcoming the permanent magnet's flux is allowed to be created through an electrical coil. When creating the reverse flux through the electrical coil, electrical energy in the form of a current is utilized. As can be appreciated, the coils have a tendency to heat when current flows through them. This invention specifically reduces the amount of current and thereby excess heat generated within the coils.
Another disadvantage of the prior art is that the mechanical energy of a hammerspring when in the printing mode causes it to return to the pole pin with a degree of excess kinetic energy. This excess kinetic energy creates a wearing effect or unwanted impact on the magnetic circuit and the steel of the hammersprings and the pole pieces upon impact. This wear has been greatly reduced by this invention through a wear bar with a hard, low friction coating at the interface between the hammerspring and the wear bar.
A further advantage of this invention is that the overall design of the frame and multiple magnetic circuits are such that there is no steel in contact with any of the neighboring magnetic circuits. Thus, magnetic interaction is very low. The frame is made from non-ferrous alloys. Potting material of a non-conducting non-magnetic nature is also used. The hammersprings can be released or fired independently in a uniform manner to avoid different densities of dots regardless of the dot's spacing densities, or the printed subject matter.
Pole pins and the creation of a magnetic circuit from a permanent magnet in the prior art was generally provided by a solid series of pins. The pole pins of this invention are made by using sheets of low hysteresis, high saturation, low carbon silicon iron laminations welded together. These laminations or sheets when welded together at the hammerspring interface help to avoid arcing and discharge which tends to pit the face of the hammerspring and the pole pieces. Additionally, the relatively soft material of the pole pieces is protected from impact by using the anti-wear bar to absorb the impact energy as referred to hereinbefore.
The welding across the shorted ends of the magnetic pole pin faces prevents arcing. Furthermore, the pole pins when made from the laminations prevent eddy currents of a nature that would require increased energy. Thus decreased energy requirements are manifest in the design through the reduction of the eddy currents within the laminated pole pins.
When the hammersprings move away from the ends of the pole pins, the pole pins are subject to a change in magnetic flux. An arcing is encountered unless they are grounded. In order to avoid the arcing, the pole pieces are grounded to the hammersprings through the frame of the hammerbank.
In order to provide for sufficient magnetic energy to retract the hammersprings and at the same time allow for release thereof by the current through the coils overcoming the permanent magnetic force, a trade-off must be established. This trade-off is established through the size of the permanent magnet, the shunt gap, the air gap and the relative permeabilities. This has been established in an optimum manner by this invention when considering the required magneto motive force (mmf) and flux densities along with the numerous permeabilities of the non-parallel air gaps.
The trade-off for the improvement of reluctance is a resulting increase in iron weight and magnetic volume. Thus, the mass of the entire printer or hammerbank as it moves is such wherein it has to be accounted for. Nevertheless, when considering the lower temperatures required, the faster reaction time and overall lessening of heat and more accurate printing, the establishment of this invention with respect to its design is a significant step over the art.
It has been found that the initial magnetic reluctance has been decreased by a significant amount with respect to the prior art non-shunt designs. Thus, when considered in light of the prior art, this invention is a significant advance thereover and will be seen to be unique and patentable as established through the specification set forth hereinafter.